High speed automotive diesel engines capable of 4500 to 5000 r.p.m. that have been in mass production are the Daimler-Benz engine or variations of the Ricardo "Comet" design. The engines have all been 2 valves; OHV or OHC design. Diesel engines have their own distinctive complications due to the high compression ratios needed to run these engines.
Valve lift near or at top dead center of the piston is nil due to the small clearance between the valves and the piston at top dead center (TDC) to prevent hitting of the valve into the piston. Because of manufacturing tolerances, both the intake and exhaust valve are designed to be effectively closed at piston top dead center.
The valve lift is adversely affected at the critical valve overlap period when the intake valve is beginning to open and the exhaust valve is closing. The limitation of valve lift at this time affects the thorough flushing of the exhaust gases and inhibits the cylinder filling process for the subsequent cycle. The reduced valve lift during the overlap period, and the long valve periods necessitate a late intake closing and an early exhaust opening. A late intake valve opening and closing reduces the effective compression ratio with detrimental starting and running consequences, and greatly reduces the trapped volumetric efficiency and compression temperature at low speeds. An early exhaust valve opening wastes energy and raises the exhaust gas and exhaust valve temperature which forces the use of more expensive and exotic high temperature valve and seat materials.
An early exhaust closing raises the probability of a recompression spike, or "lock-up" at TDC during the scavenging or overlap portion of the cycle at high speed and high load, when in some engines, there is not sufficient real time available for a complete evacuation of the exhaust gases. Recompression spikes, apart from inhibiting the proper gas-flow process and reducing volumetric efficiencies and power output consume energy by creating negative work on the exhaust stroke near TDC. The exhaust valve closing must occur late enough during an extended overlap period with the intake valve to prevent a recompression spike near top dead center.
Diesel engines have been able to tolerate these problems at low speeds. The operation at low speed provides sufficient time for the air flow through the intake and exhaust valves to pass into and out of the cylinder even with a delayed intake valve opening or early exhaust valve closing. However, the problems associated with valve timing and air flow lag become magnified at high speeds. The combination of a late intake opening and an early exhaust closing provides for increased risk of a recompression spike at high speed operation. However the high compression ratios of a conventional high-speed I.D.I. diesel engine with the piston at top dead center being very close to the valves dictate that the intake valves cannot be opened early due to crashing into the piston and the exhaust valve cannot be closed late due to the crashing of the piston into the exhaust valve. The unnatural valve timings detract from the potential high-speed capability of the diesel engine.
A major compromise of these prior-art high-speed, 2-valve engines results when the intake valve opening must be delayed until the piston reaches TDC. In every case, the intake valve closes excessively late in the compression stroke, and the effective compression ratio, effective compression pressure and effective compression temperature are too low even for the high speeds.
When such engines run at low speeds, the same applies, but in addition, the volumetric efficiency suffers because the upward piston motion on the compression stroke "spit-back" into the intake manifold the air which has already been admitted into the engine and for which energy has been spent. Negative work (more energy wasted) also results from returning certain amounts of this already-admitted air back into the intake manifold. The situation is further aggravated at cranking speeds, especially cold when the batteries are weak and the oil is thick and said speeds are in the order of 100-150 rpm. The effective compression pressure and temperature under said conditions is lowered so much that cold startability is greatly affected or impossible.
The lower effective compression ratios are also the main reason why diesels have the distinctive knock when they idle. The compression of the air charge does not achieve ignition temperature conditions until late in the cycle when all fuel from the injector has been introduced into the combustion chamber. The ignition results in an uncontrolled explosion of all the fuel, practically at the same time, with the resulting distinctive diesel bang or knock. A combustion process is desired in which higher compression temperature is achieved at points near piston TDC. The incoming injected fuel will ignite in a shorter period of time (chemical delay time), achieving ignition after only a smaller portion of the fuel charge is injected and burning the remaining portion of the fuel in a controlled manner as injection proceeds, producing not only a smoother, quieter combustion, but also lower firing pressures and NO.sub.x levels.
Certain designs have unsuccessfully attempted to overcome the problems of the close approach of the piston to the cylinder head. Some engineers have attempted to sink the valves into the cylinder head. This design has been unsuccessful. Firstly, the cylinder head shrouds the opening of the valve such that inadequate air flow results when the valves are beginning to open. If the cylinder head is cut back to eliminate the shroud, the size of the combustion chamber is then increased which undesirably lowers the compression ratio.
Modifications to pistons have also increased efficiency of engines. Many engines have a piston with a recess to form part of the combustion chamber or to enhance air swirl. The "Comet" diesel engine have a "spectacle-shaped" recess in its piston to form the main active combustion chamber. The chamber is not aligned or coordinated with the valves to act as a pocket to increase the clearance between the valves and the piston at TDC. Nissan has developed an engine in which valve pockets exist in the piston. The pockets allow the valve heads to protrude into the combustion chamber (rather than into the head) to eliminate the air flow shrouding effects at low valve lifts.
Divided combustion chambers, also referred to as indirect injection engines, have a separate "pre-combustion chamber" or "pre-chamber" as it is generally known, in direct communication with the cylinder through at least one passage. Air enters it from the cylinder during the compression stroke. The fuel is injected into the pre-chamber towards the end of the compression stroke as the piston nears TDC. The fuel mixes with the highly turbulent air in the pre-chamber at high velocity created by the passage of air through the relatively small transfer passage. After an appropriate delay period, the fuel ignites and the mass of burning fuel and air is then expelled back into the main chamber at high velocity where it mixes with the rest of the air in the main chamber for the main combustion phase.
Two advantages occur with pre-chamber designs. Firstly, the tail ends of injection are assimilated much better by indirect injection designs. The tail ends of fuel injection often results in large size droplets. The large size droplets have less surface area in which to mix with air in order to completely burn. To further complicate matters, the large size droplets also have less time to completely burn because they are the last of the fuel to be injected. As such the inadequacies of the fuel injector cause much soot and smoke by incompletely burning the tail ends in an open chamber design. The pre-chamber design more completely breaks up the large droplets from the tail ends of injection by the intense air mixing in the pre-chamber and the very high temperatures within the pre-chamber. In the early days of the diesel engine, when "solid" fuel injection by mechanical means was introduced, divided chamber engines made possible the application of compression-ignition principles to relatively small engines, such as trucks and buses. Part of the reason was because of the second advantage of pre-chambered engines: the ability to run at speeds higher than was then customary with bigger industrial or marine engines. The pre-combustion chamber design of the time, by violently mixing and quickly burning the fuel, in spite of the very poor ignition characteristics of the fuel systems of the time, allowed engines to run up to 1500 rpm; sometimes 1800 rpm, which made possible the introduction of smaller cylinder sizes (down to 2 liter/cylinder) typical of truck and bus engines, and later, cylinder sizes of less than 1 liter which first allowed diesel engine installation in passenger cars, small boats, and small construction equipment.
However, divided chamber designs have certain inherent drawbacks. Firstly, the separate pre-chamber increases the overall surface to volume ratio of the hot part of the combustion chamber thus increasing the thermal losses which increases fuel consumption. Secondly, the pumping of the gases into the pre-chamber and out of the pre-chamber costs energy. Thirdly, the high heat losses must be compensated in order to achieve self ignition temperature of the fuel, especially during engine start-up. These drawbacks are addressed in the form of higher compression ratios. Pre-chamber designs often are approximately 22.5:1. The high compression ratios require extremely close manufacturing tolerances in all the major engine components. Even slight variations can have gross and detrimental effects on the nominal compression ratio of the engine. In practice, the tolerances may cause significant differences from engine to engine and, within the same engine, from cylinder to cylinder causing uneven and rough performance. Another disadvantage of divided chamber engines is that the torch-like jet of flame exiting the pre-chamber and entering the main combustion chamber would in some cases impinge directly on the piston top at or near TDC. Special measures such as the use of high temperature steel heat dams on the piston tops, or oil cooling jets shooting oil into the bottom of the piston to cool down the piston temperature coated by the very hot flame jet add to the expense and complication of the diesel engine. Even with these measures, many piston tops undergo heat checking and thermal cracking.
What is needed is a high-speed diesel engine with highly improved power output with lower fuel consumption, improved startability and reduced combustion noise and harshness, and offering increased durability of valves and piston. The process takes advantage of, and is based on, appropriate recesses incorporated in the piston which, apart from functioning as active combustion chambers, also provide for valve pockets to receive the intake and exhaust valves for unique and improved valve timing without combustion or manufacturing compromises, and which contribute to an even thermal loading. The combination of volumetric efficiencies and valve timings providing previously unheard of startability, smooth, quieter combustion and reduced firing pressures, even while producing increased power outputs.